The present invention relates to a developer composition for photographic elements comprising photosensitive coatings on substrates, or more particularly to developer compositions for removing the non-image areas of exposed negative working lithographic printing plates.
In the past, various methods of developing printing plates have been employed. Such methods include the use of compositions containing organic solvents, surfactants, salts, acids and other art recognized materials in solution.
It is most desirable to have a developer composition which is capable of completing development in a short period of time. The oleophilic image areas of the printing plate should be ink receptive and water repellant whereas the hydrophilic non-image areas should be water receptive and greasy ink repellant.
The proper selection of organic solvents as components of the developer solution is very important. If the selected solvent is a good solvent for the light sensitive composition itself, the solvent will not satisfactorily distinguish between the exposed areas and the unexposed areas. As a result, a large portion of the image area coating is dissolved in the solvent or is severely swollen and removed. Such printing plates have no practical value. By the addition of a large amount of water to reduce the concentration of the organic solvent in the developer solution, image differentiation can be improved. However, during the development of a printing plate, particularly negative working plates, nonimage areas comprising resin binders such as acetals, and particularly polyvinyl formal, solvate in the developer solution and separate from the support and contract to spherical shapes in a highly tacky condition. Such solvated particles deposit upon still-solvent-wet image areas and adhere permanently even after drying. Not only are these "redeposited" areas visible to the naked eye on the developed printing plate but are removed by tacky ink during printing which pulls away the true underimage, leaving skips which show as absence of image on the printed sheet.
To provide a partial solution to the problem, volatile solvents are used in many prior art developer compositions. It is expected that the use of such volatile solvents in the developer solution will prevent tackiness in the image by permitting rapid evaporation. However, use of large amounts of water soluble low boiling point solvents for example, as used in the developer described in U.S. Pat. No. 3,954,472, produces the problems of working sanitation, hazards due to low flash point, and pollution. A reduction of the concentration of these water soluble low boiling point solvents helps to minimize these problems in the working environment, however, the speed of development will be adversely affected. Also the tackiness of the coatings is reduced but not completely eliminated.
To improve the speed of development of developer solutions containing a lower concentration of low boiling point solvents, an adjuvant organic solvent is sometime added. Unfortunately, such developers suffer from several drawbacks including: toxicity, developer instability, odor or inability to effectively clean out unwanted background non-image areas.
Lithographic printing plates generally are composed of an aluminum containing substrate which may or may not have been treated with a variety processes recognized in the art process including anodization, graining and hydrophilization. The thusly prepared substrate may then be applied with a photosensitive coating comprising a photosensitizer, binding resins, colorants, acid stabilizers, surfactants and other art recognized components. Common photosensitizers include diazo compounds, including polymeric diazonium condensates salts. Sensitizers, binders and printing plates employing aromatic diazonium compounds are described in U.S. Pat. Nos. 3,175,906; 3,046,118; 2,063,631; 2,667,415; 3,867,147 and 3,679,419.
Typical prior art developer compositions are described in U.S. Pat. Nos. 2,754,279; 4,381,340 and German OLS No. 2,216,419.
The present invention improves upon the aforementioned drawbacks of other lithographic developers.
It is an object of this invention to provide a developer for lithographic printing plates having resin binders in their photosensitive coatings, which will enable the operator to prepare a plate substantially free from objectionable redeposit, and further having a reduced toxicity and higher flash point while simultaneously achieving commercially acceptable removal of non-imaged areas.